We report a case of an 11-year-old boy who underwent successful bone marrow transplantation for X-linked hyper-IgM syndrome (XHIM). The donor was an HLA-matched brother. The patient was conditioned with busulfan, cyclophosphamide and anti-thymocyte globulin. He received 4.7 x 10(8) marrow cells per kg from the donor. Prophylaxis against graft-versus-host disease consisted of cyclosporine and short-term methotrexate. The clinical course after the bone marrow transplantation was uneventful, and 12 months after transplantation the patient was doing well with no need for therapy. We examined expression of the CD40 ligand (CD40L) on the patient's activated T lymphocytes and in vitro production of immunoglobulins by his lymphocytes. Although expression of CD40L was totally absent before the bone marrow transplant, subnormal expression appeared after the transplantation. In vitro production of IgG and IgA also was improved by the transplant. Based on our experience bone marrow transplantation appears to be a reasonable therapeutic option for patients with XHIM if HLA-matched family donors are available.